An OLED (organic light-emitting diode) is a light-emitting diode (LED), in which the emissive electroluminescent layer is a film of an organic compound, which emits light in response to an electric current. A typical OLED has a multi-layer structure, and typically includes an indium tin oxide (ITO) anode, and a metal cathode. Sandwiched between the ITO anode and the metal cathode are several organic layers, such as a hole injection layer (HIL), a hole transfer layer (HTL), an emitting material layer (EML), an electron transfer layer (ETL), and an electron injection layer (EIL).
Several properties required for such electroluminescent and charge transport compounds include high fluorescent quantum yield in solid state, high mobility of electrons and holes, chemical stability during vapor-deposition in vacuum, and the ability to form stable films. Common problems with OLEDs include fast aging/short life span, undesirably high operating voltages, or insufficient efficiency. The art recognizes the on-going need for new materials that enable longer device life span, lower operating voltages, and greater efficiency. The art recognizes the need for such light-emissive materials for the hole transport layer of OLEDs in particular.